The invention is in the field of surface acoustic wave (SAW) devices. It relates to improving the accuracy of such devices, and to a crystal having an angular orientation and a propagation direction which provide particularly advantageous compensation for stress and temperature effects in such devices.
SAW devices, such as resonators and delay lines, are used in sensors for measuring parameters such as acceleration, stresses or strains, and pressure, in communications and in other fields. SAW device sensors generally are based on the propagation of surface acoustic waves across a thin, flexible diaphragm which is deformed when subjected to an applied acceleration, stress or strain, or pressure. They measure the effects of the parameter of interest on properties of the surface acoustic wave and use this measurement to deduce the parameter of interest. In communications and in other fields, surface acoustic wave devices are used in a variety of signal processing applications. Various uses of SAW device sensors are discussed in commonly owned U.S. Pat. Nos. 4,419,600 granted on Dec. 6, 1983 for a Stress-Compensated Quartz Resonator and 4,512,198 granted on Apr. 23, 1985 for Surface Acoustic Wave Sensors, and in co-pending commonly owned patent applications Ser. No. 427,240 (filed on Sept. 29, 1982) and Ser. Nos. 687,715 and 687,716 (both filed on Dec. 31, 1984). Said commonly owned patents and co-pending patent applications are hereby incorporated by reference in this specification, as though fully set forth herein, and the Examiner's attention is directed to the background portions of their disclosures, and to the prior art discussed or cited therein. See, also, U.S. Pat. Nos. 3,771,072; 3,772,618; 3,818,382, 3,866,153; 3,995,240; 4,109,172; 4,109,173; 4,220,888; 5,224,548; 4,224,549 and 4,323,809.
One of the desirable and important characteristics of SAW devices is their frequency stability. However, frequency stability can be adversely affected by environmental factors, which are manifested primarily in various types of biasing states or stress distributions in the surface along which the acoustic wave propagates. For example, temperature-induced strain and acceleration-induced body forces, in conjunction with mounting support forces, can result in an undesirable frequency drift. A need exists to make SAW devices more immune to environmental factors, and important aspect of the invention are directed to meeting at least some aspects of that need.
It has been recognized that the selection of crystalline orientation and direction of propagation of surface acoustic waves in a SAW device can have a significant effect on the way environmental factors affect frequency stability. While it may be possible to deduce a desirable crystalline orientation and direction of propagation with respect to certain environmental factors, for example through the use of simulations involving linear relationships of wave motion, there are certain environmental factors which are not in that category. For example, temperature-induced effects and stress-induced effects on the surface wave propagation are believed not to be susceptible to a solution based on linear equations of motion. Nevertheless, temperature effects and stress effects can be of significant importance for frequency stability.
One configuration which is believed to be widely used in the art involves using quartz plates in the so-called ST-cut. Another uses the so-called SST-cut. Using the standard adopted by the Institute of Radio Engineers, now the Institute of Electrical and Electronic Engineers or IEEE which appear in "Standards on Piezoelectric Crystals, 1949: Standard 49 IRE 14.S1." Proceedings of the IEEE, December 1949, pp. 1378-90, the ST-cut is characterized by an angular orientation theta of 42.75.degree. and propagation direction gamma of 0.degree., and the SST-cut is characterized by an angular orientation of -49.22.degree. and propagation direction gamma of 23.degree.. It is believed that the commonly used ST-cut SAW devices do not provide adequate compensation (at least in some applications) for stresses such as planar, isotropic stress distributions in the plane of the device, or for biaxial stresses due to a flexural loading of the substrate, and provide compensation only over a very narrow bandwidth for periodic, thermal stress distributions due to an array of metallic electrode stripes. It is also believed that ST-cut SAW devides are significantly affected by dynamic thermal effects. Similar, but not identical, shortcomings are believed to be associated with SST-cut SAW devices. See, also, U.S. Pat. No. 4,499,395 which proposes certain other cuts.
One aspect of the invention is based on the discovery that unexpected significant improvement in stress compensation and temperature compensation is SAW devices can be achieved by using a different cyrstalline orientation and a different propagation direction than is known to have been used before for such devices. In particular, an important aspect of the invention is the discovery of a new cut, called herein the STC-cut (standing for stress and temperature compensation cut), characterized by an angular orientation and a propagation direction different from those known to have been used in the prior art for such devices. A SAW device utilizing the STC-cut provides, as compared with an ST-cut device, high SAW velocity, lower viscous attenuation, lower dynamic thermal effects, greater compensation for planar isotropic stress in the plane of SAW propagation and for biaxial stresses due to flexural loading, and compensation over a larger bandwidth for periodic thermal stress distribution due to an array of electrode stripes. In an exemplary and nonlimiting embodiment, the STC can be characterized by a propagation direction gamma=46.9.degree. ma=46.9.degree. and an angular oriengation theta=41.8.degree., using the IEEE standard notation referred to above. Other important aspects of the invention are discussed in the detailed description below.